Metal inserts are commonly used to enhance the structural and/or functional performance of plastic parts These inserts are usually hollow and are also often internally threaded. They may be installed by ultrasonically or electrically heating the insert, to thus melt the adjacent plastic, and then pressing the insert into a prepared hole in the plastic part while the plastic defining the hole is molten. These inserts have been designed with knurls, undercuts, fins and the like on their external surfaces which are subsequently filled with melted plastic during installation to thus provide resistance to tension and torque. In addition, sealing rings and/or adhesive have been used to make sure that a leakproof pressure tight seal will be established between the outside surface of the insert and the surface of the hole in the plastic.
More specifically, these inserts are installed (utilizing heat or ultrasonic/vibrational energy) into holes which were formed during the part molding process Installation is achieved by heating the insert above the melting point of the plastic and by applying a downward force to the insert. As the insert is pushed into the molten plastic defining the hole, the remelted plastic flows around the outside of the insert filling the knurls and undercuts. When the plastic cools and hardens it effectively anchors the insert in place. However, when the plastic cools it shrinks slightly leaving a small gap or gaps between the insert and the parent plastic material resulting in an assembly which exhibits leakage between the insert outside diameter and the plastic parent material.